[0001] The present invention relates to a transparent rear-projection screen of the kind
which on its rear side facing projectors is provided with a set of lenses for paralleling
the light coming from the projectors and which on its front side is provided with
horizontally spaced apart lenses in the form of upright ridges of triangular cross
section and convex lenses between the ridge-shaped lenses.
[0002] Rear-projection screens have to a great extent been used for video-projection devices,
micro film readers, data machines and flight simulators. Screens of this kind are
known from U.S. Patent No. 4,509,822 issued April 9, 1985.
[0003] It is known to produce a projected TV-picture by turning three projectors, each of
them with their own ground colour (red, green and blue), towards a transparent projection
screen.
[0004] The three projectors are usually positioned next to each other horizontally, and
enlarged pictures produced by the projectors are projected towards the transparent
screen. Because the three projectors are placed next to each other, their optical
axes form angles with each other. Normally the optical axes form angles with each
other from 7° to 12° depending on the size of the picture tubes and the distance from
the picture tubes to the transparent screen.
[0005] Most designers of projection-TV place the green picture tube between the red and
the blue picture tubes so that the optical axis of the green picture is projected
perpendicular to the screen. This has the effect that the optical axes of the blue
and the red picture tubes deviate with e.g. 9° in comparison with the optical axis
of the green picture tube.
[0006] If, on the TV concerned a plain mat screen is applied, an observer watching the screen
slantwise from the front will see a picture which is either blue or green dominant,
depending on whether the observer is closer to the optical axis for the projectors
giving a red or a blue picture. This problem is called "colour shadow".
[0007] According to the known technique, the lenses on the front side of the rear-projection
have been designed so that they are total refracting, that is, the light from the
projectors - after having been paralleled, is directed to the flanks of the ridge-shaped
lenses in the direction perpendicular to the major plane of the screen and is then
refracted out through the top and flanks of the ridge-shaped lenses. With such a known
technique, it is difficult to eliminate the problem of the "colour shadow".
[0008] In the known rear-projection screen, part of light beams of green colour incident
upon the screen normally thereto from the green picture tube is totally reflected
by the respective flanks of each ridge-shaped lens and emitted to the front side of
the screen through the respective opposite flanks of the ridge-shaped lens. For this
reason, the brightness characteristic of the green light as viewed towards the front
surface of the screen is that there are two maximum values of brightness in regions
offset symmetrically in two opposite directions with respect to an axis of zero-degree
angle of observation towards the front surface of the screen. For each of the red
and blue colour lights from the picture tubes, there are also two maximum values of
brightness. As stated before, the three colour picture tubes have their optical axes
forming angles with each other, so that the maximum values of brightness of the three
colours appear in mutually shifted fashion due to the difference in the angle of incidence
of the lights from the picture tubes. This means that when the screen is observed
from the front side at various angles to the screen, the brightness of each colour
changes so that the colours shadow is produced.
[0009] The object of the present invention is to remove the "colour shadow" problem for
an observer watching the picture within approx. ±30° horizontal range in comparison
to normal to the screen, i.e., within the horizontal range where the convex lenses
operate.
[0010] According to the present invention, the above object is attained by a transparent
rear-projection screen of the above stated kind in which each of the ridge-shaped
upright lenses has on its flanks light diffusing means.
[0011] The light diffusing means may be irregularities of the surfaces of the flanks, matted
surfaces of the flanks, or a half-refracting coating on the flanks.
[0012] The present invention will be described in more detail below with reference to the
drawings, in which:
Fig. 1 is a schematic plan view showing three projectors and a rear-projection screen
to which light rays are emitted from the projectors;
Fig. 2 is a horizontal section of a rear-projection screen of the present invention;
Fig. 3 is a fragmentary enlarged view of Fig. 2, showing a first embodiment of the
present invention;
Fig. 4 is a view similar to Fig. 3 but showing how green and blue rays of light are
reflected and refracted;
Fig. 5 is a view similar to Fig. 3 but showing a second embodiment of the present
invention;
Fig. 6 is a view explaining how the light beams pass through the rear-projection screen;
and
Figs. 7, 8 and 9 show three different examples of practice constructed according to
the present invention, respectively.
[0013] Referring to Fig. 1, there are illustrated three projectors 1, 2 and 3 which project
TV-pictures towards the rear side of a projection screen 4. These projectors emit
green, red and blue light, respectively. The three projectors are placed next to each
other horizontally, so that the middle projector 2, most often the green projector,
has its optical axis perpendicular to the screen 4. The screen 4 consists of Fresnel
means 4a for paralleling light from the projectors 1, 2 and 3 and lens means 4b for
refracting the paralleled light.
[0014] The optical axes of the projectors 1 and 3 often form an angle of 6°-l0° with the
optical axis of the projector 2. By means of the three projectors 1, 2 and 3, as well
as projection lenses 5, 6 and 7 mounted in front of them, it is possible to form -
proportional to the projectors - an enlarged picture on the screen 4.
[0015] According to the known technique, standing to the right or to the left of the screen's
centre line O-O, the observer will see a red or a blue dominating picture, respectively,
depending on whether the observer is closer to the red or the blue projector's optical
axis. An observer being opposite the centre line of the screen will see the screen
red dominating at the right side, if this side has the red projectors optical axis
directed to the right with respect to the screen's centre line O-O.
[0016] It is this colour uncleanness or colour shadow, for an observer watching the picture
from the front, that the present invention aims to eliminate.
[0017] As shown in Figs. 2 and 3, the rear-projection screen 4 has a body 11 of e.g. an
acrylic resin, having a rear side with Fresnel lenses 15 facing the projectors. These
lenses 15 function to parallel the light from the projectors and direct the light
to a front side of the screen 4 in a direction normal to the major plane of the screen.
The front side of the screen is provided with a number of horizontally spaced lenses
14 in the form of upright or vertical ridges of a triangular cross section. On the
both sides of each ridge-shaped lens 14, there are provided convex lenses 12 and 13.
The Fresnel lenses 15 correspond to the means 4a shown in Fig. 1 and the lenses 12,
13 and 14 to the lens means 4b in Fig. 1.
[0018] In Fig. 3 the light beam 17 from the projectors is converted by the Fresnel lenses
15 into a beam 17a normal to the major plane of the screen 4. The beam 17a is directed
to the side flanks 18 and 19 of the ridge-shaped lens 14 which has an acute angle
β.
[0019] According to an improvement of the present invention, the side flanks 18 and 19 of
each ridge-shaped lens 14 are coated with a half-reflecting layer 22 of lacquer of
light diffusing nature. The lacquer has a refractive index equal to or higher than
that of the material of the screen 4. The thickness of the layer 22 is from 2 to 3
µ. A wax is added to the lacquer in order to achieve a desired light diffusion. CaCO₃
is further added to the lacquer for achieving a desired light diffusing in the layer
22.
[0020] As a consequence of the light diffusion, the light beam 17a is divided into two vector
bundles 20 and 21, one bundle 21 passing out through the layer 22 on one flank 19
and the other bundle 2u being produced from light reflected by the flank 19. The vector
bundle 21 is deflected at an angle α relative to the normal to the surface of the
flank 19 and normally at an angle α/4 relative to the surface of the flank 19. In
order to make sure that the angle α/4 is 15°, the angle β must be more than 30°, but
less than 43°, preferably 37°, entirely depending on the applied lacquer's index of
refraction and its dulling medium mixed. It is to be noted that the vector bundles
20 and 22 going out of the screen 4 include rays of light having mutually deflected
directions, which neutralize the change of colour within an angular range of ±10°
relative to the normal. This phenomenon is based on the light diffusing function of
the layer 22 of lacquer, which causes mixing and intersecting of rays of light.
[0021] Fig. 4 shows the path of rays of light from two projectors, red (R) and blue (B)
respectively. The projectors have in Fig. 4 a mutual angle of 16° between their optical
axes. When the rays have passed the coated layer 22, the red and the blue rays (R,
B) are mixed.
[0022] Fig. 5 shows another embodiment of the present invention in which the flanks 18 and
19 of the lens 14 are caused to have irregular surface shape or a sinusoidal surface
22A. The pitch of the waves of the sinusoidal surface however should not exceed three
times as much as, and should be greater than the mean value of the longest wavelength
of the light. Because such an irregular or sinusoidal surface has a number of normals
in different directions relative to the major plane of the flanks 18 and 19, a variety
of refracted rays of light 20 and 21 having deviated directions are obtained which
neutralizes the change of colours. It will be understood that in Fig. 5 the irregularities
are shown exaggerated and are actually minute ones.
[0023] The irregular surface of the flanks 18 and 19 may be a mat surface. The mat surface
is preferably made with a tool made with an artificial diamond, which, due to its
granular structure, will leave an irregular surface identical to the size of the grains
on the diamond. The minute irregularities of the matted surface will split up or mix
refracted rays of light and eliminates dominating preferential direction of the light
beam of each colour.
[0024] When the tool has been thus manufactured, it gets surface treatment described below.
There are two alternatives:
A: In a container containing a chemical fluid, aluminium oxide with a thickness of
10-20 µm is applied (gives a slightly mat surface), and is then sealed.
B: In a container with the chemical fluid, the tool is corroded for 10-20 seconds,
then aluminium oxide with a thickness of 10-20 µm is applied, and is finally sealed.
[0025] The advantage of the processes A and B are as follows:
A: The surface of the tool becomes hard.
B: The surface prevents inconvenient reflections from the outside (front side) of
the screen.
[0026] The surface treatments A and B will give the same result as a coating.
[0027] The screen 4 may include a light refracting medium mixed evenly therein or in one
of the sides of the screen. The medium is an organic or inorganic pigment which serves
to eliminate the colour shadow.
[0028] In Fig. 6, a division of the path of rays of green light through the upright lenses
12, 13 and 14 is shown. As can be seen in Fig. 6, the ray G₃ will not be deflected,
because the surface, where the ray leaves the acrylic screen 4, is at right angle
to the normal to the surface. However, the rays G₁, G₂ and G₄ will undergo a deflection
in accordance with index of refraction for the concerned screen material. If radius
of curvature of the lenses 12, 13 is increased further, these lenses will involve
total reflection. The path of rays in the lens 14 has previously been described, and
it is evident in Fig. 6 that the lenses 12, 13 and 14 will supplement each other in
such a manner that the viewing angle will be approx. 150°.
[0029] Fig. 7, 8 and 9 show practical examples of rear-projection screens according to
the present invention.
Example 1
[0030] By means of a tool or mould with a profile as illustrated in Fig. 7, a 3 mm thick
plate of PMMA with 20 g SiO₂ per m² and with a grain size of 5-35 µm was cast. The
distance D between the tops of the lenses 14 was 0.80 mm. The radius R of curvature
of the lenses 12 and 13 was 0.30 mm, the angle β was 38°. Moreover, the distance d
between the flanks 18 and 19, where these meet radius of curvature of the lenses 12
and 13, was 0.26 mm. Further, the angle γ₁ - between the radius and the tangent for
the lenses 12 and 13, where these meet the lens 14 - was 90°. After being cast, the
screen was coated with a lacquer, index of refraction 1, 50, and 25 g wax as well
as 15 g CaCO₃ per liter lacquer was added.
[0031] The screen showed the following parameters:
Peak gain: 5.6
1/2 horizontal peak gain: 38°
1/2 vertical peak gain: 7.5°.
[0032] Peak gain means the direct transparent light measured as normal to the surface in
comparison to a known reference (MgCO₃).
[0033] The screen showed good efficiency so that the picture was made visible over a big
visual angle, considerably exceeding ±75°. Further the screen showed an extreme colour
unity.
Example 2
[0034] In this example the design as illustrated in Fig. 8 was applied, showing that the
lenses 12 and 13 are not an entire sector of a circle, but both have plane surfaces,
where these meet each other. This design is suitable for projection-TV in particular,
where the optical axes between the projectors differ more than 8°. Process of manufacture
was very much identical to the one mentioned under example 1.
[0035] The following parameters were measured:
Peak gain: 5.8
1/2 horizontal peak gain: 36°
1/2 vertical peak gain: 8°.
[0036] In order to further increase the diffusion of the light in both vertical and horizontal
direction for rear-projection screens, it is advisable to add a diffusing medium.
Such a medium could e.g. be SiO₂, CaCO₃, BaSO₄ as well as fine powdered glass with
an index of refraction 0.05-0.07 larger or smaller than the basic material, which
could be an acrylic resin (Polymethyl Metacrylate) with an index of refraction of
1.49.
Example 3
[0037] In Fig. 9 is shown a design with the following specifications: angle β of the lens
14, 40°; angle γ₁ = 80°; R = 0.15 mm; Distance D = 0.40 mm; γ₂ = 50°; d = 0.14 mm.
The significant modifications in this design, compared to previous examples, is the
modification of the angle γ₁ from 90° to 80°. This modification will subdue the direct
transparent light, i.e. light within the measuring angular range of ±8°, so that the
screen will now have a lower peak gain, but a more even distribution of light.
[0038] The screen indicated the following values:
Peak gain: 4.2
1/2 horizontal peak gain: 36°
1/2 vertical peak gain: 8°.
[0039] This screen showed the best colour purity of all 3 examples.
1. A transparent rear-projection screen of the kind which on its rear side facing
projectors is provided with a set of lenses (15) for paralleling the light coming
from the projectors and which on its front side is provided with horizontally spaced
apart lenses (18) in the form of upright ridges of a triangular cross section and
convex lenses (12, 13) between the ridge-shaped lenses (14), characterized in that
each of the ridge-shaped lenses (14) has light diffusing means (22, 22A) on its flanks
(18, 19).
2. A screen according to claim 1, wherein the light diffusing means is a half-reflecting
layer (22) of lacquer applied to the surfaces of the flanks (18, 19) of the ridge-shaped
lenses (14).
3. A screen according to claim 2, wherein said layer (22) is mixed with wax and CaCO₃
and the index of refraction of the wax and CaCO₃ differs from that of the material
of the screen.
4. A screen according to claim 1, wherein the light diffusing means is an irregular
surface (22A) formed on the flanks (18, 19).
5. A screen according to claim 1, wherein the irregular surface (22A) is a matted
surface.
6. A screen according to claim 1, wherein a light refracting medium is mixed in the
screen.
7. A screen according to claim 6, wherein the light refracting medium is evenly distributed
in the screen or in one of the sides of the screen.
8. A screen according to claim 7, wherein the light refracting medium is an organic
or inorganic pigment.